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Origin of Venus’ atmosphere
Figure 3.14: Image of Venus obtained by the Pioneer Venus space probe in the ultraviolet (1979). This image shows clearly the presence of the Y-shaped pattern of clouds. (Image: NASA/JPL website).
During the formation of the solar system, planetesimals in primitive orbits swept through space, collecting dust and debris and later accreting gas. The dust were usually composed of a rocky core wrapped in ice from diverse volatiles. During the accretion process there was a gravitational energy release, which add to the energy that comes from unstable radioactive isotopes. Due to the protoplanet rotation, which acts as a huge centrifuge, the planet suffers an internal differentiation process in layers, depending on its constituents density (Turcotte, 1995).
During the differentiation process into core, mantle and crust, volatiles sought way to the surface, remaining either in liquid form (as is the case of the oceans and superficial water on Earth), or in gaseous form (thus providing the planets’ atmospheres). Another differentiation process consists of altering themselves chemically or by interaction with solar radiation, leading to the appearance of new molecules, some of them which such low mass that easily reach escape velocity and leave the planet’s gravitational sphere of influence (this is the case of water molecule in Venus atmosphere, after enduring a photolysis process and breaking up into hydrogen and oxygen (Donahue, 1997)).
The present Venus atmosphere
Venus has an atmosphere dominated by carbon dioxide. Its atmospheric dynamics are essentially driven by the solar thermal heating and its very low rotation rate, the year (224.7 Earth days) is shorter then a Venus day (243.02 Earth days) (Shapiro, 1968). As a consequence from its low rotational velocity, the Venus’ atmospheric dynamics major mechanisms are quite different from Earth. Instead of a quasi-geostrophic regime that rules its global atmospheric motions, is the equilibrium from the pressure gradient force that consists in the exact equatorward component of the centripetal force (the cyclostrophic balance, that prevails at slow rotating planets) that imposes its system of horizontal winds, parallel to equator, the Zonal Winds. That fact turns Venus’ meteorological research even more interesting If in the one hand Venus is a “CO2 dominated” planet, on the other hand, its atmospheric balance is fine tuned by the SO2 concentration (among other minor components) at this evolutional stage of present atmospheric conditions (Bullock and Grinspoon, 2001).
Global atmospheric circulation
There has been an important effort from the planetary scientific community in order to better understand the atmosphere’s global circulation, with many studies based on recent data collected by Venus Express and ground-based observations, as well as a large modelling effort using global circulation models of Venus (GCM). Among several onboard different experiments, measurements of the zonal wind (horizontal wind, i.e. parallel to the equator moving anticlockwise, from east to west) were made by tracking cloud features in images at ultraviolet, visible and infrared wavelengths (Markiewicz et al., 2007; Sánchez-Lavega et al., 2008) and are particularly relevant to inter-comparison with the ground-based methods I developed over the course of this thesis. Figure 3.31: Venus atmosphere’s global circulation. Figure: Taylor and Grinspoon (2009). Venus atmospheric dynamics is essentially driven by thermal heating and by its low rotation rate. In spite of all the present efforts to better understand the atmospheric global circulation, the explanation of this phenomenon is still essentially based on the phenomenological evidence. There is currently no theory that can fully constrain the structure and the dynamics of the atmosphere.
In the atmospheric circulation at lower and middle atmosphere there is a pronounced convection mechanism at the low latitudes. Midlatitudes are characterized by the zonal wind (RZW) flowing in bands almost parallel to equator. At 50-60 a jet is sometimes detected, already found by previous missions to Venus Express but highly unstable. On the polar region close to the polar axis was found a wide binary vortex (also highly variable) structure covering several km2 and turning on itself with a period of around two and half terrestrial days (figure 3.31).
There are three major atmospheric global circulation processes that characterize the Venus atmospheric dynamics (see figure 3.31 and for more detail the figure 3.32). Mesospheric circulation (between the altitudes of 60-100 km and stretching between mid-latitudes) most relevant atmospheric motion is the super-rotational retrograde zonal wind (RZW) flowing in quasilaminar bands parallel to equator. Super-imposed to the previous air motion stands out the sub-solar to anti-solar circulation (SS-AS) transporting the over heated air from high insolation regions towards the nightside radiation deficit area. The third dynamical macro regime is the meridional circulation, responsible for the transport of the heat excess from low latitudes, poleward to cooler high latitudes regions, see the atmospheric circulation scheme from figure 3.32. Meridional circulation is characterized by one cell in each hemisphere, with rising equatorial heated air in a highly convective region, then an upper branch of the Hadley cell drive this hot air till high latitudes where it will sink and return to low latitudes along the Hadley cell lower branch (Limaye, 1985).
Table of contents :
Aknowledgements
Abstract
Résumé
Resumo
Figure inde
Table index
1 Thesis overview
1.1 Scientific context
1.2 Objectives
1.3 Thesis structure
2 Introduction
3 Venus: Earth’s paradoxical twin
3.1 The planet Venus
3.1.1 Geologic evolution
3.1.2 Venusian volcanism
3.2 History of Venus exploration
3.2.1 20th century exploration
3.2.2 Present Venus exploration
3.2.3 Future scheduled missions
3.3 Atmospheres of telluric planets
3.3.1 Genesis of the atmospheres in telluric planets
3.3.2 Brief notions of atmospheric dynamics
3.4 The Venus atmosphere
3.4.1 Origin of Venus’ atmosphere
3.4.2 The present Venus atmosphere
3.4.3 Global atmospheric circulation
3.4.4 Venus atmospheric evolution
3.4.5 Brief comparative climatology
4 VLT/UVES observations
4.1 UVES – Ultraviolet-Visual Echelle Spectrograph
4.2 VLT and UVES description
4.2.1 Telescope light path
4.2.2 Opto-mechanical design
4.2.3 High resolution spectroscopy with UVES
4.2.4 High-precision wavelength calibration
4.3 Observations with UVES
4.3.1 Objectives
4.3.2 Description of Observations
4.3.3 Data
4.3.4 Problems that affected the observations
4.4 Doppler velocimetry with UVES
4.4.1 Doppler shift geometric projection factor
4.4.2 Young effect
4.5 Sensitivity Tests
4.5.1 Synthetically reconstructed spectra tests
4.5.2 Fully synthetic spectra tests
4.6 Results (UVES)
4.6.1 Slit parallel to rotation axis
4.6.2 Slit perpendicular to rotation axis
4.6.3 Analysis of spatial variations
4.7 Discussion (UVES results)
5 CFHT/ESPaDOnS observations
5.1 The CFHT telescope and the ESPaDOnS instrument
5.1.1 The Canada-France-Hawaii-Telescope (CFHT)
5.1.2 ESPaDOnS – Instrument Description
5.2 Observations with CFHT/ESPaDOnS
5.3 Doppler velocimetry with ESPaDOnS
5.3.1 Projected radial velocities
5.3.2 Velocity extraction
5.3.3 Young effect correction
5.3.4 Monitoring the instrumental drift
5.3.5 Error estimate
5.3.6 Altitude of observations
5.3.7 Doppler velocimetry analysis
5.3.8 Observing geometry correction
5.3.9 Kinematical wind models
5.4 Results (ESPaDOnS)
5.4.1 De-projected wind circulation
5.4.2 Zonal wind variable component
5.5 Cloud tracking VEx/VIRTIS-M analysis
5.5.1 Instrumental description
5.5.2 VEx/VIRTIS-M observations
5.5.3 Cloud tracking analysis
5.6 Comparing Doppler velocimetry and cloud tracking
5.6.1 Simultaneous velocity measurements with CFHT/ ESPaDOnS and VEx/VIRTIS
5.6.2 Comparison with previous visible Doppler velocimetry measurements
5.6.3 Comparison with previous CT measurements
5.7 Discussion (ESPaDOnS results)
6 Observing missions
6.1 The 2012 Venus transit
6.1.1 Preparation
6.1.2 Transit event
6.2 Observing Venus with TNG/NICS
7 Conclusions and outlook
7.1 Conclusions
7.2 Future work
7.3 Final remarks
Publications
